In medical practice, there are many situations in which it is necessary to obtain information about the condition of the circulatory system. In particular, clinical medicine often requires certain parameters so that decisions can be made about specifically influencing the cardiovascular system, for instance, to decide whether it would be advisable to replenish the circulatory system with an infusion solution, or whether—as an alternative or to what extent—the circulatory system should be assisted by means of circulation-active drugs. The parameters of the heart-lung interaction (HLI) have proven to be especially useful in this context. These parameters, however, are usually based on an invasive measurement of the arterial blood pressure and require laborious cannulization of an arterial vessel.
Non-invasive methods for measuring HLI parameters have not been very satisfactory so far because of their poorer signal quality. The pulsatile signals, that is to say, the pressure changes in the arterial vessels caused by the pulse, are measured directly over the tissue using an external device. In this process, damping effects, caused by damping in the tissue and in the measuring device, have a considerable negative impact on the signal-to-noise ratio. This is why non-invasive methods fail in actual practice,
U.S. patent application 2005/187481 A1 puts forward methods for determining HLI parameters. Mention is made of a non-invasive measurement employing finger clips, arm cuffs or ear clips. However, it is not disclosed how a suitable signal quality can be achieved with these measuring instruments.
U.S. Pat. No. 5,255,686 discloses a device for non-invasive, continuous blood-pressure measurement. The measuring component is a conventional blood-pressure cuff with which the systolic, diastolic and mean blood pressures are determined.